Signing you in

Every time I clone a repo, I have to figure out how to run it.

This is fine for my own projects. But when I'm reviewing a PR, onboarding a new teammate, or setting up CI for a polyglot team, it becomes noise.

rex is a single Go binary that detects your project's stack and runs the right command. No config file. No reading the README.

git clone https://github.com/someone/unknown-project cd unknown-project rex test # it just works Enter fullscreen mode Exit fullscreen mode How it works under the hood The detection logic is surprisingly simple — and that's the point.

This happens in <50ms because it's just os.Stat calls — no network, no parsing heavy files.

Once the stack is known, rex maps standard verbs to the right tool:

// Go test -> "go test ./..." run -> "go run ./cmd/server" build -> "go build ./..." // Node (pnpm) test -> "pnpm test" run -> "pnpm run dev" build -> "pnpm run build" Enter fullscreen mode Exit fullscreen mode Step 3: Priority chain (the smart part) rex doesn't blindly guess. It respects what's already there:

1. Justfile / Makefile (task runner overrides everything) 2. package.json scripts / Cargo.toml / go.mod (ecosystem native) 3. Language heuristics (fallback) Enter fullscreen mode Exit fullscreen mode If a Makefile defines a test target, rex test runs make test — even in a Go project.

The most interesting part is how this became a GitHub Action.

Instead of requiring Go to be installed in the runner, the action downloads the correct binary directly from the release page:

- uses: rexrun-dev/rex@v0.4.0 with: command: test Enter fullscreen mode Exit fullscreen mode This single step replaces 10+ lines of stack-specific YAML. Works for all 12 supported languages.

I used GitHub's composite action type instead of Docker. Why?

The trade-off? The install script has to handle OS/arch detection:

OS=$(uname -s | tr '[:upper:]' '[:lower:]') ARCH=$(uname -m) case "$ARCH" in x86_64) ARCH="amd64" ;; aarch64|arm64) ARCH="arm64" ;; esac URL="https://github.com/rexrun-dev/rex/releases/download/${VERSION}/rex_${VERSION#v}_${OS}_${ARCH}.tar.gz" curl -sL "$URL" | tar xz -C /usr/local/bin rex Enter fullscreen mode Exit fullscreen mode Watch mode A recent addition: rex watch polls the filesystem and re-runs your command on change.

rex watch test # re-run tests when files change rex watch build # re-build on change Enter fullscreen mode Exit fullscreen mode It's not using inotify or fsnotify — just a lightweight polling loop with hash-based change detection. This makes it portable across all platforms without CGO or platform-specific dependencies.

rex ci generates a GitHub Actions workflow file customized for your detected stack:

$ rex ci ✓ created .github/workflows/ci.yml (go project) Enter fullscreen mode Exit fullscreen mode The generated YAML uses the official setup actions for each language (setup-go, setup-node, setup-python, etc.) and runs the correct test command.

brew tap rexrun-dev/tap && brew install rex # or go install rexrun.dev/rex/cmd/rex@latest Enter fullscreen mode Exit fullscreen mode Or try the interactive playground at rexrun.dev — paste any GitHub repo URL and see what rex would detect.

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